1. Field of the Invention
This invention relates to arc resistant metal clad switchgear and particularly to switchgear having upper and lower compartments that are mirror images of one another and arc resistant doors to the compartments. The doors lock against a division sheet between the compartments.
2. Background Information
Metal-clad switchgear design is the first choice design of most consultants and utilities, particularly for medium voltage switchgear operating at voltages of from 5 to 38 KV. Circuit breakers and/or other elements in the switchgear are housed in metal enclosures and are removable. Drawout apparatus are used to move the circuit breakers or other elements between a disconnect position in which the primary contacts are fully disengaged and a connect position in which the primary contacts are fully engaged. The switchgear typically includes a variety of auxiliary devices and related control circuitry, including potential transformers, power transformers and fuse drawer compartments.
The metal-clad design for switchgear concentrates on structure that reduces the possibility of arcing faults within the enclosure. For instance, all primary elements such as breakers, voltage transformers and control power transformers have disconnect means with isolating shutters establishing isolation from the high voltage source.
All operations on breakers, potential transformers and control power transformers, engagement/isolation/test in metal-clad switchgear are preferably performed with the compartment doors closed for operator safety. The front door is interlocked with the shutter assembly to reduce the chance of accidental opening of the front door during even partial levering-in of the breaker/potential transformer/control power transformer drawout element.
The switchgear enclosures containing primary elements have been compartmentalized and grounded for maximum isolation and confinement such as the breaker compartment, main bus compartment and cable compartment. Within these compartments all live parts where possible are fully insulated reducing the possibility of an arcing fault to occur. This need to eliminate or reduce the possibility of an arcing fault has driven the design development to metal-clad switchgear construction for many years.
Although arcing faults are rare, injuries from arcing faults in metal-clad switchgear have continued. When it does occur, the results can be very destructive because of the energy levels reached within a confined compartment. The structural containment may be inadequate to prevent arcing products and hot gases escaping the faulted compartment. Burns can result if operating personnel are in close proximity to the faulted switchgear.
Arcing faults can occur within a switchgear compartment as a result of insulation failure or human error. The pressure from an electric arc is developed from two sources: the expansion of the metal in boiling, and the heating of air by the arc energy. Copper expands by a factor of 67,000 times in vaporizing. This accounts for the expulsion of near-vaporized droplets of molten metal from the arc. One test has shown that droplets could be propelled up to 10 feet. The pressure also generates plasma outward from the arc for distances proportional to the arc energy. One cubic inch of copper vaporizes into 1.44 cubic yards of vapor. The air in the arc stream expands in warming up from its ambient temperature to that of the arc temperature (approximately 35,000.degree. F). All this happens within the first half cycle of the fault and results in a sudden, large rise in pressure inside of the compartment. Switchgear structure is needed to reduce arcing and contain such pressures and to prevent personal injury.
There are several known designs for metal-clad switchgear to allow the doors to withstand the pressure generated from an internal arcing fault, some of which are:
The doors are stiffened, using welded angles and channels and then bolted using many bolts around the perimeter of the door into weld nuts in the cell; PA1 Multi point latch handles that when turned rotate pins or bars into hasps that brace the doors around the perimeter; PA1 A special handle assembly that as the handle rotates the door is either pushed up or down causing mating notches to overlap--this overlapping of the notches gives the door latching system the ability to withstand the pressure; and PA1 A handle that turns a three point latch which extends steel pins into steel pads welded onto one side of the cell in combination with special hinges on the opposite side of the door.
Some medium voltage switchgear include upper and lower switchgear compartments and separate doors for the compartments. An improved switchgear enclosure and door latch system is needed for providing structural containment of pressure that may develop from possible arcing within the switchgear enclosure.
There is need, therefore, in switchgear having an improved latching mechanism for containing pressures that may result if arcing occurs in the switchgear.
There is a more particular need for improved switchgear having upper and lower fuse truck compartments that are mirror images of one another and doors on the compartments that provide improved arc resistance.
There is an additional need for such improved medium voltage switchgear which provides a more efficient and convenient arrangement of compartments and especially for an improved door latch system for the switchgear.